POP namelist variables that are controlled by XML

### Description of the issue:

There are several variables in POP that the user should not be able to change via `user_nl_pop`, and many of them are ecosystem-related.

Low-hanging fruit:
```
abio_dic_dic14_on
cfc_on
ciso_on
ecosys_on
iage_on
irf_on
sf6_on
```
are all set according to what's in `OCN_TRACER_MODULES`

@klindsay28 [expanded the list](https://github.com/marbl-ecosys/MARBL/issues/130#issuecomment-276564139) with

> Here are some (probably more to come later):
> chl_option depends upon OCN_CHL_TYPE
> lactive_ice depends upon OCN_ICE_FORCING
> atm_co2_opt depends upon OCN_CO2_TYPE
> lvariable_PtoC will depend upon OCN_TRACER_MODULES_OPT

This is not just a MARBL driver issue, so I'm not applying any labels

### Version:
- CESM: 2.2
- POP2: cesm_pop_2_1_20190410
- Migrating from [marbl-ecosys/MARBL#130](https://github.com/marbl-ecosys/MARBL/issues/130), originally noted in January 2017 and assigned to @mvertens because at the time she was working on a python-based `build-namelist` script. I'm not assigning her this ticket because I don't believe that work has continued.

### Machine/Environment Description:

Machine independent

### Any xml/namelist changes or SourceMods:

Problem exists out of the box